The invention relates to a weldable aluminum product comprising a structural component in the form of sheet or plate product or an extruded product, for use as a structural member in a welded structure. Further the invention relates to a welded structure comprising at least one such aluminum product. Such welded structures include marine structures e.g. fixed structures, ships, aerospace vehicles, and land vehicles such as trucks, dump trucks, cars (automobiles) and railway vehicles. While in the following description of this invention reference is mainly made to aluminum sheet or plate products, it is to be understood that products based on aluminum extrusions are also included. As is conventional, the term xe2x80x9caluminum productxe2x80x9d refers to aluminum alloy products.
When welding aluminum plates or sheets by means of fusion welding, such as MIG, TIG or plasma welding, in particular plates or sheets made of the Aluminum Association AA5xxx-series alloys having Mg as an alloying element in the range of 2 to 6 weight %, for example welding head-to-head by using a V-bevel, a weld bead is formed having a relatively sharp angle between the base material and the weld toe. In a severe and undesirable case even an undercut may be formed. Usually fatigue cracks start at a so-called hot-spot, which is the notch where the weld bead starts. Because of this relatively sharp angle or notch several organizations setting standards, e.g. for ship-building, indicate that the fatigue behavior of the welded construction detail is essentially independent of the aluminum alloys used, but is dependent on the construction notch effect. This becomes more apparent for the AA5xxx-series alloys having a relatively high Mg content. For this reason an aluminum alloy having improved strength may still be assessed on its fatigue performance as if it were a less strong aluminum alloy, because of the notch effect. And therefore the acceptable xe2x80x9cglobal stressesxe2x80x9d in a welded construction detail are reduced to lower levels than the aluminum alloys used would withstand in themselves.
It is known to improve the conformation of the weld bead zone after welding, e.g. by hammer preening or weld re-melting, but such an extra step should be avoided if possible. There is therefore a need for a product providing an improved weld bead in the welding process.
In the context of disclosure of the invention below, some other prior art documents are here mentioned, though they are not concerned with the problem described above.
WO98/28130 describes providing a clad layer of filler material on an aluminum core sheet, for the purpose of providing a filler for a welding pool.
In light structures, not assembled by welding together of components, it is known to provide a clad layer on a core sheet, the clad layer having a corrosion potential lower than that of the core sheet. GB-A-1004868 describes materials for use in domestic and industrial water heaters having low corrosion performance, particularly illustrated by an AA6061 alloy sheet clad with an AA7072 alloy layer containing up to 0.01% Cu. Weldability is not mentioned. GB-A-1416134 describes alloys of the AA4xxx-series clad with an AA7072 layer to give corrosion resistance. Formation of tubes of this material by welding is mentioned, but weldability of the material is not discussed.
An object of the invention is to provide a weldable aluminum product for use as a structural member in a welded structure, which gives the welded structure an improved fatigue life.
To achieve this object there is provided in accordance with the invention a weldable aluminum product comprising a structural component which is a sheet, a plate or an extruded body and is made of an aluminum alloy containing not more than 1.5 weight percent (wt %) Zn and, adhered on at least one side of said structural component, a cladding layer made of an AA7xxx-series alloy having a corrosion potential lower than that of said alloy of said structural component.
With this product there is achieved the effect of obtaining after welding a much smoother transition from the parent plate or sheet to the weld metal than has been known before in welding of aluminum alloys, in particular of AA5xxx-series alloys. In the weld there is a higher barrier for the crack initiation stage with a resulting increase in fatigue life, and thus a fatigue limit can be achieved which may be considerably higher than that for welded joints known so far. Furthermore, this smoother transition overcomes the need for additional modification techniques of the weld toe profile, such as local grinding or weld toe remelting techniques. Furthermore, it has been observed that the flow behavior of the molten metal during welding is improved, resulting in a reduced amount of undesirable inclusions in the weld metal, such as oxides.
The use of the clad layer or layers in the invention can not only improve the xe2x80x9chot spotxe2x80x9d of the welded joint, but may also produce compressive stress at the surface of the welded structure, because of the difference in contraction of the structural component and the clad layer. This may reduce residual tensile stress due to contraction of the weld bead and thus also influence fatigue behavior positively.
An additional advantage of welding the cladded aluminum product of the invention is that surface compressive residual stresses may be introduced in the regions where fatigue cracks are likely to initiate, which may extend fatigue life significantly. By applying a cladding on one or both sides of the product, additional laborious techniques of introducing residual stresses, e.g. peening, can be overcome.
For certain applications due to the cladding there will be advantages such as visual improvement and better forming (shaping) behavior because the relatively soft surface avoids crack initiation. Furthermore, the weldable cladding allows for higher strength tempers to be used for the alloy of the structural component because of the improved corrosion behavior due to the anodic protection given by the cladding used.
Although the invention may be applied to all kinds of aluminum alloys as the structural member having a corrosion potential higher than that of the cladding applied, the effects found are most pronounced in AA5xxx-series alloys having Mg as an alloying element in the range of 2 to 6 weight percent. The amount of Zn is preferably 0.4 to 1.5 weight percent.
Preferably the thickness of the structural component or core is in the range of up to 40 mm, more preferably in the range of 0.5 to 20 mm.
Preferably the thickness of the or each cladding layer is in the range of up to 20% of the thickness of the structural component. The minimum thickness is preferably 1% of that of the structural component, and more preferably the thickness of the or each cladding layer is 1 to 15% of the thickness of the structural component, and most preferably 1 to 10% of the thickness of the structural component, more particularly 2 to 10%.
Adhesion between the cladding layer or layers and the structural component (core sheet or core plate) is very important to obtain structural integrity after welding. Preferably the adhesion is obtained by rolling. Suitable roll-bonding processes are known. Alternatively, the adhesion is obtained by casting a composite ingot having simultaneously cast contacting portions of respectively the materials of the structural component and the cladding layer or layers, followed by rolling of the ingot obtained. This provides a mainly oxide free interface. In the case where the structural component is an extruded body, the cladding layer may be applied by any suitable method, for example roll-bonding.
In one embodiment of the aluminum product in accordance with the invention the structural component is made of an AA5083-series alloy, an AA5059-series alloy, an AA5086-series alloy, or an alloy which is a modification of one of these.
A very advantageous AA5xxx-series alloy for the structural component has the following composition, in weight percent:
balance aluminum and inevitable impurities.
In this embodiment with the structural component clad with a AA7xxx-series alloy, a smooth angle between the parent component and the weld toe of 130xc2x0 or more can be obtained, which allows for application in welded constructions designed for fatigue strength, since with the smoother weld groove the fatigue properties of the welded construction details come close to the welded fatigue properties of the parent material, as tested in SN-curves.
Another very advantageous AA5xxx-series alloy for the structural component has the following composition, in weight percent:
balance aluminum and inevitable impurities.
Preferably, in the aluminum product in accordance with the invention, the cladding layer is made of an AA7xxx-series alloy comprising zinc in a range of 0.3 to 5.0 weight %, more preferably in a range of 0.3 to 2.5 weight %, and more preferably made of an AA7072-series alloy. It has been found that by applying an AA7xxx-series alloy as cladding having zinc in the indicated range, an improved corrosion protection is provided to the heat-affected zone in particular. The heat-affected zone in Mg-containing alloys in particular has usually the worst microstructure with respect to corrosion resistance due to the high precipitation density, which limits the application of the welded structure obtained in particular in higher temperature applications, typically above 80xc2x0 C. The cladding remains solid and essentially undisturbed during welding in the region near the heat-affected zone. Thus, after welding, a good anodic protection is obtained for the critical heat-affected zone due to the presence of the zinc. Additionally, the cladding provides an anodic corrosion protection to the structural component which is not affected due to the welding.
AA7072 alloys have the following composition, in weight percent:
balance Al and inevitable impurities.
In another aspect of the invention there is provided in an aluminum welded structure having at least two aluminum alloy members joined by welding, at least one of the members being an aluminum product of the invention as described above.
Typical welded structures to which the invention is applicable include parts of marine transportation vessels such as catamarans of monohull type, fast ferries, high speed light craft, fixed marine structures, aerospace vehicles, land transportation vehicles such as cars, trucks, lorries and railway vehicles and silos and armor plate.
Suitable welding techniques which may be applied include TIG, MIG, laser and electric arc welding, and plasma welding.
Various suitable filler metals may be applied during the welding depending largely on the composition of the structural component of the weldable aluminum product, and include AA5183 and AA5087 in the case where the structural component is made from an AA5xxx-series alloy.